netdev - Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL

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Date:   Sat, 21 Dec 2019 12:08:43 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Maor Gottlieb <maorg@...lanox.com>
Subject: Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL_PIN

On Fri, Dec 20, 2019 at 03:54:55PM -0800, John Hubbard wrote:
> On 12/20/19 10:29 AM, Leon Romanovsky wrote:
> ...
> >> $ ./build.sh
> >> $ build/bin/run_tests.py
> >>
> >> If you get things that far I think Leon can get a reproduction for you
> >
> > I'm not so optimistic about that.
> >
>
> OK, I'm going to proceed for now on the assumption that I've got an overflow
> problem that happens when huge pages are pinned. If I can get more information,
> great, otherwise it's probably enough.
>
> One thing: for your repro, if you know the huge page size, and the system
> page size for that case, that would really help. Also the number of pins per
> page, more or less, that you'd expect. Because Jason says that only 2M huge
> pages are used...
>
> Because the other possibility is that the refcount really is going negative,
> likely due to a mismatched pin/unpin somehow.
>
> If there's not an obvious repro case available, but you do have one (is it easy
> to repro, though?), then *if* you have the time, I could point you to a github
> branch that reduces GUP_PIN_COUNTING_BIAS by, say, 4x, by applying this:

I'll see what I can do this Sunday.

>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bb44c4d2ada7..8526fd03b978 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1077,7 +1077,7 @@ static inline void put_page(struct page *page)
>   * get_user_pages and page_mkclean and other calls that race to set up page
>   * table entries.
>   */
> -#define GUP_PIN_COUNTING_BIAS (1U << 10)
> +#define GUP_PIN_COUNTING_BIAS (1U << 8)
>
>  void unpin_user_page(struct page *page);
>  void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>
> If that fails to repro, then we would be zeroing in on the root cause.
>
> The branch is here (I just tested it and it seems healthy):
>
> git@...hub.com:johnhubbard/linux.git  pin_user_pages_tracking_v11_with_diags
>
>
>
> thanks,
> --
> John Hubbard
> NVIDIA